Probe separates sound waves from turbulent flow pressure fluctuations in ducted airstreams by using principle that sound waves and turbulent flow pressure perturbations travel at different velocities

Moore, M. T.

Smith, E. B.

English

NASA Technical Reports Server

November 1975	 B75-10286 
NASA TECH BRIEF 
Lewis Research Center 
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Sound Separation Probe 
mask the existing noise levels over much of the low 
frequency portion of the sound spectrum. Therefore, 
REFERENCE 
PRESSURE TUBE probe pressure measurements can be misleading as to the 
amount of acoustic energy that is actually propagating 
along the duct and radiating to the surrounding 
atmosphere. 
The sound separation probe shown in Figure 1 senses 
the pressure fluctuations simultaneously at two closely 
spaced points with flush-mounted pressure transducers. 
The time-averaged signals at the two measurement points 
on the probe will be similar and each represents the usual 
probe measurements. However, the instantaneous signals 
will not be alike due to the different propagation times of 
sound waves and turbulent flow noise between the two 
measurement points. By using standard cross-correlation 
analysis, the signal from the aft transducer on the probe is 
compared to the time-delayed signal from the forward 
transducer to determine the amount of similarity between 
the two signals. This analysis results in a graph of the 
similarity between the two signals as a function of time 
delay between them. A typical cross-correlogram for 
sound mixed with turbulence in a flowing duct is shown 
in Figure 2. The cross-correlation function displays two 
peaks. The first peak is the portion of the signal 
ELECTRICAL 
CONNECTORS 
SIN. SIL TUBE 
.5600D x .060 WALL 
S
SIN. SIL TUBE 
• 375 CD x 024 WA LL 
1.00 
TRANSDUCERS
4.5,,
Figure 1
A probe has been developed that is able to separate 
sound waves from turbulent flow pressure fluctuations in 
ducted airstreams by using the principle that sound waves 
and turbulent flow pressure perturbations travel at 
different velocities. 
In order to understand the propagation of sound waves 
from a device such as a jet engine, it is necessary to make 
pressure measurements in the ducted airstream. These 
measurements are made with probes in the ducts to 
determine what type of sound deadening devices can be 
applied to the duct walls to absorb the sound energy. 
The problem with making probe measurements is that 
the fluctuating pressures sensed by the probe are not 
exclusively associated with sound. Turbulence due to the 
flow of air causes pressure perturbations which frequently
CROSS
CORRELATION
COEFFICIENT 
PEAK DUE TO SOUND
	 I PEAK DUE TO TURBULENCE 
TIME DELAY 
Figure 2
(continued overleaf) 
S 
This document was prepared under the sponsorship of the National 	 Government assumes any liability'resulling from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19750000286 2020-03-19T20:17:43+00:00Z
Patent Status: 
NASA has decided not to apply for a patent. 
SOUND MIXED 
WITH TURBULENCE 
/ 
SOUND SEPARATED 
FROM TURBULENCE 
I	 I	 I 
2	 3	 .4	 5 
FREQUENCY KHZ 
Figure 3
Source: M.T. Moore and E.B. Smith

General Electric Co.

under contract to

Lewis Research Center
(LEW-1 2507)
propagating at the speed of sound relative to the flow. 
The second peak is the portion of the signal corresponding 
to the flow turbulence. The ratio of the two peaks 
together with the overall level of the signal is then used to 
determine the level of the sound separated from the 
turbulence. The results of separating various levels of 
sound of a typical spectrum is shown in Figure 3.
Note: 
No additional documentation is available. Specific 
technical questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B75-10286 
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B75-10286	 Category 03 


Sound separation probe

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